The present invention relates to a protective system for a high voltage transmission line, and more particularly, to improved polarizing and operating signals for phase selector and distance measuring relays for the protective system.
Protective systems for high voltage transmission lines sometimes employ phase selector relays and distance measuring relays for detection of fault conditions within the protected portion of the transmission lines. The phase selector for a particular phase A, B, or C, of the transmission line may be connected to the secondary phase-to-ground voltage of the transmission line. Each particular phase selector should only detect faults related to the particular phase. For example, a phase selector relay A should only detect phase A-to-ground faults, phase A-to-B faults or phase C-to-A faults. Furthermore, the phase selector for phase A should not be affected by a phase B-to-ground fault, a phase C-to-ground fault, or a phase B-to-C fault. The single phase discriminating detection operations are desired so that a fault related to a single phase will not cause a three-phase circuit breaker to be activated.
Further, it is desired that the protection of the transmission line remain secure or stable for unfaulted conditions. For example, during the open pole time or period which the faulted phase or phases are disconnected from the transmission line, it is desired that the phase selector and distance relays detect and respond to a fault which may develop on the unfaulted phases of the transmission line.
In addition, it is desired that the phase-selector device does not operate under maximum line-loading conditions, and it is preferable that the phase selector operates only for faults in the selected zones of the transmission line.
The parameter of the phase selectors are commonly selected to correspond to parameters of the desired reach. For example, the phase selector parameters of forward reach may correspond to the forward distance of the protected line. The above discussed preferred detection of faults within the protected line and discriminating detection operation of a phase selector relay are also applicable to distance measuring relays employed in protective systems.
The selected parameter for detecting faults determine, among other things, the operating characteristic of the phase selector and distance measuring relay. The operating characteristics are further determined by comparators, such as phase-comparators, which detect the phase coincidence between operating and polarizing signals, that are responsive to the normal or abnormal conditions existing within the transmission line. Exemplary phase comparator circuits are described in U.S. Pat. Nos. 4,034,269 and 4,161,011 both of which are assigned to the same assignee of the present invention.
The accuracy of each of the phase selector relay and distance measuring relay is directly dependent upon the accuracy of the phase comparator detector, which, in turn, is dependent upon the accuracy of the operating and polarizing signals which detect the presence of normal or abnormal conditions within the desired zone of the transmission line.
Accordingly, it is an object of the present invention to develop operating and polarizing signals for a distance measuring relay that accurately determine the normal and abnormal conditions existing in a desired zone or zones of a transmission line and for phase selector relays that accurately determine the phase or phase directly involved in the abnormal condition.
It is another object of the present invention to provide phase selector and distance measuring relays adapted to be coupled to a high voltage transmission line and which operate in such a manner that a fault related to a single phase of the high voltage transmission line does not cause multiple phase tripping.
It is a further object of the present invention to provide protection of the transmission line that remains secure for the unfaulted conditions.
It is a still further object of the present invention to provide a system comprised of a phase selector relay and a distance measuring device in which the distance measuring relay operates under control of the phase selector. The effect of the system is to reduce the total electrical equipment necessary to implement both the phase selector relay and the distance measuring relay while also improving the performance of each relay.
These and other objects of the present invention will become apparent to those skilled in the art upon consideration of the following description of the invention.